Miniature pumps, hereafter referred to as micropumps, can be constructed using fabrication techniques adapted from those applied to integrated circuits. Such fabrication techniques are often referred to as micromachining. Micropumps are in great demand for environmental, biomedical, medical, biotechnical, printing, analytical instrumentation, and miniature cooling applications. Just as in larger-scale applications, various pump designs are required for different micropump systems. For certain applications in which space is at a premium, pumps with minimal dimensions, particularly pump cavity dimensions, are of interest. Reduced-dimension pumps which substantially maintain performance levels of larger dimension pumps are particularly desirable.
Micropumps may include active, passive or fixed valves. Active valves have components that are actuated or otherwise driven or moved and include, for example, solenoid-actuated valves or magnetically or electrostatically driven control valves. Passive valves also have movable parts as exemplified by movable polysilicon check valves. Both passive and active valves have limited effectiveness when used to pump fluids containing particulates. These valves can become obstructed or fail to seal when particulates are present.
Fixed valves are valves having no moving parts (no-moving-parts valves, NMPVs) and represent the utmost simplicity and high reliability for pumping fluids. Such valves, which do not include parts that periodically seal and move apart, are especially advantageous for micropump systems used for pumping fluids that include particulates, for example liquids that contain particles. Fixed-valve pumps are particularly useful for biological applications that require pumping fluids that contain cells. A mechanism exists for cell damage in moving-parts valves that is not present in pumps with fixed valves. Smaller diameter pump cavities and valve conduits (sized from 10's to 100's of .mu.m) of NMPV micropumps can be produced by application of etching techniques in silicon wafers and by application of micromolding techniques for fabrication of micropumps from a variety of plastics or related materials.
The effectiveness of fixed valves can be characterized by the parameter "diodicity," which is the ratio of pressure drop in the reverse-direction fluid flow through the valve to the pressure drop in the forward-direction fluid flow through the valve, for a given flow rate. A basic design consideration for a fixed-valve micropump is valve configurations that result in a diodicity greater than 1.0. In this regard, the small size of such valves, and the very low flow range (from 100's of nL/min to 1,000's of .mu.L/min, for example) will typically yield a relative low Reynolds number (less than about 200-400) which is a dimensionless parameter that is proportional to the product of the valve size and flow velocity and inversely proportional to the kinematic viscosity of the fluid. Accordingly, valve configurations for use in micropumps must effect the requisite diodicity in flows characterized by low Reynolds numbers, where turbulence (with attendant significant pressure losses) is unlikely to occur.
Several fixed valve configurations have been demonstrated to function in low Reynolds number micropumps. Stemme and collaborators have reported a planar double chamber pump with diffuser/nozzle valves fabricated in silicon using an isotropic HNA-etch (A. Olsson et al. (1995) "A valve-less planar pump in silicon," 8th Int'l Conference on Solid- State sensors and Actuators-Eurosensors IX, Digest of Technical Papers (IEEE Cat. No. 95TH8173) vol. 2 P.291-294, Found. Sensors & Actuator Technol., Stockholm, Sweden; A. Olsson et al. (1996) "A valve-less pump isotropically etched in silicon," J. Micromech. and Microeng. (UK) 6(1):87-91; G. Stemme (1995) "Microfluid sensors and actuators," Proceedings of the Sixth Int'l Symposium on Micro Machine and Human Science, p. 45-52, IEEE New York, N.Y.) A maximum pump capacity of 230 .mu.l/min and maximum pump pressure of 1.7 m H.sub.2 O were reported for a diffuser/nozzle pump with 6 mm diameter pump chamber operated for pumping methanol at a resonance frequency of 1318 Hz. The same group later reported a similar two-chamber (6 mm diameter) micropump with diffuser/nozzle valves fabricated using deep reactive ion etching (DRIE) with a maximum pump pressure of 7.6 m H.sub.2 O and maximum pump flow of 2.3 ml/min for water (A. Olsson et al. (1996) "An improved valve-less pump fabricated using deep reactive ion etching," Proceedings IEEE The Ninth Annual Int'l Workshop on Micro Electro Mechanical Systems, (Cat. No. 96CH35856) p. 479-484, IEEE New York, N.Y.; A. Olsson et al. (1997) "Micromachined flat-walled valveless diffuser pumps," J. Micromech. Systems 6(2):161-166). WO 94/19609, (Stemme and Stemme) pub. September 1994, relates to diaphragm displacement pumps employing diffuser/nozzle valves.
The Stemme et al. group had earlier reported single chamber and double chamber minipumps employing diffuser/nozzle valves fabricated in brass (G. Stemme et al. (1993) "A valveless diffuser/nozzle-based fluid pump," Sensors and Actuators A A39(2): 159-167; A. Olsson et al. (1995) "A valve-less planar fluid pump with two pump chambers," Sensors and Actuators A A47 (1-3):549-556.
T. Gerlach et al. (1995) "A new micropump principle of the reciprocating type using pyramidic micro flowchannels as passive valves," J. Micromechan. Microengin, 5(2):199-201 reports a micropump using direction-dependent fluid dynamic behavior as dynamic passive valves. Pyramid-shaped microchannels (narrowest d=123 .mu.m with length=370 .mu.m) and a 10 mm.times.10 mm.times.(0.4-0.75) mm pump chamber are fabricated by anisotropic KOH etching in a silicon wafer. The authors report a pump flow rate of 260 .mu.L/min when the pump is operated in a "quasi-static" mode at frequencies of 100 Hz to 1 Hz and report a pump flow rate of 480 .mu.L/min when the pump is operated at a frequency of 8 kHz. T. Gerlach et al. (1995) Sensors and Actuators A A50(1-2):135-140;WO 96/00849, (Gerlach et al.) pub. January 1996, relates to micropumps employing pyramid-shaped microchannels as valves.
Forster et al. (1995) "Design, fabrication and testing of fixed-valve micro-pumps," Proceedings of the ASME Fluids Engineering Division 1995 ASME Int'l Mechanical Engineering Congress and Exposition FED-Vol. 234 p.39-44 reports micropumps with NMPV valves etched on silicon wafers. Two types of valves were tested: diffuser/nozzle valves and valvular conduits (also called fluid rectifiers) with branched channels which have more restricted flow in one direction in the valve. Valvular conduits for high Reynolds number systems are described in U.S. Pat. No. 1,329,559 (Tesla) and U.S. Pat. No. 5,265,636 (Reed). U.S. patent application Ser. No. 08/401,546, filed Mar. 9, 1995 (Afromowitz et al. ) is, among other things, directed to micropumps with NMPVs. This application describes diffuser/nozzle valves, valvular conduits and conduits with curved sidewalls for redirecting reverse flow which all function as NMPVs.
The present invention is generally directed to methods of fabrication of improved micropumps that employ fixed inlet and outlet valves and to micropumps with improved performance and characteristics compared to those currently known in the art.